1. Field of the Invention
The present invention relates to a light-emitting device, and, more particularly, to a light-emitting device having a plurality of emission layers.
2. Description of the Background Art
In recent years, displays using organic electroluminescent devices (hereinafter referred to as “organic EL devices”) as flat-panel display devices have been developed to find applications in a wide variety of information equipment. The organic EL devices are expected to provide flat-panel displays that consume less power than CRTs conventionally and generally used in the art, and are also expected to provide nonpolluting (i.e., mercury-free) illumination devices, which will replace fluorescent lamps, and the like.
In the organic EL device, electrons and holes are injected from an electron injecting electrode and a hole injecting electrode, respectively, into an emission layer, so that the electrons and the holes are recombined in the emission layer so as to bring organic molecules into the excited state. The organic EL device then emits fluorescent light when the excited organic molecules return to the ground state.
In recent years, an organic EL device including a plurality of emission layers adapted to emit different wavelengths of light has been proposed in, for example, Japanese Patent No. 3287344. The organic EL device disclosed in this patent includes a first emission layer that emits orange light, and a second emission layer that emits blue light. The orange light and blue light emitted by the respective emission layers are mixed together to produce a white emission.
Recently, an improvement of the luminous efficiency of the organic EL device has been desired to make the device suitable for practical use. In the case where the white emission is changed or converted through color filters to provide full-color emissions, in particular, the luminous efficiency needs to be further improved in view of an optical loss caused by the color filters.
However, the organic EL device disclosed in the above-mentioned Japanese Patent No. 3287344 may suffer from a problem that each of the first emission layer that produces an orange emission and the second emission layer that produces a blue emission reduces the luminous intensity of light emitted by the other emission layer. In this case, the luminous efficiency may deteriorate. If the organic EL device has a low luminous efficiency, a large amount of current is required to pass through the device, and the device may degrade earlier than expected, resulting in a reduction in the device lifetime.
Also, it is difficult for the organic EL device disclosed in the above-mentioned Japanese Patent No. 3287344 to improve the color purity of blue color and orange color (or red color) due to interference between the blue emission produced by the second emission layer and the orange emission produced by the first emission layer.